the atlas radiation dose measurement...

The ATLAS Radiation Dose Measurement System

Jochen Hartert

Graduiertenkolleg Seminar

Freiburg, 15 October 2008

Jochen Hartert The ATLAS Radiation Dose Measurement System 1 / 23

ATLAS

The ATLAS Experiment

Proton-proton collisisions at√

s = 14 TeV and L = 1034cm−2s−1


ATLAS Radiation Environment at LHC and SLHC

Motivation and Measured Quantities

Why care about radiation dose levels?

Irradiation changes the performance of detectors and electronics!

Measured quantities:1 Total Ionizing Dose (TID)

Mainly due to photons, electrons and positronsMeasured in Gray (Gy), 1 Gy = 1 J/kgProblem to MOS and bipolar devices

2 Non Ionizing Energy Loss (NIEL) / equivalent fluence (Φeq)

Hadrons cause displacement damage in siliconExpressed in 1 MeV neutron equivalent fluence (n/cm2)

Φeq = κΦ =R

D(E)φ(E) dE

D(En=1 MeV)

3 Thermal neutron fluence



Radiation Field in ATLAS

Exposure of electronics to:

radiation from pp-collisions(mainly pions)neutrons from interactions ofhadrons with detector material

After 10 years of LHC operationelectronics irradiated up to:

Total Ionizing Dose:TID > 100 kGyNon Ionizing Energy LossΦeq > 1015 1 MeVn/cm−2

Monitoring of radiation levelsneeded in order to:

cross check simulationsunderstand change in detectorperformanceand as independent measurement

Non Ionising Energy Loss in theATLAS Inner Detector

FLUKA simulation by Ian Dawson



Radiation Field at SLHC

Luminosity: L(SLHC) ≈ 10× L(LHC)

Ionizing dose scales with luminosity:

TID(SLHC) ≈ 10× TID(LHC)

Upgrade of ATLAS tracker to full silicon→ loss of moderating effect of theTransition Radiation Tracker→ NIEL not expected to scale withluminosity→ as compensation introduce a 5 cmthick moderator

Non Ionising Energy Loss atthe SLHC

Ian Dawson


Online Measurement Techniques in Use Total Ionizing Dose

Total Ionizing Dose (TID) Measurement - RadFETs

RadFET: Radiation Field Effect Transistor

Electrons escape, holes are trapped in SiO2-Siboundary.

Higher negative gate voltage needed to opentransitor.

Measure gate voltage increase at given draincurrent. ∆V = a× (TID)b

Sensitivity depends on oxide thickness

Three RadFETs used in ATLAS to cover largerange of doses:

0.01 Gy to 10 Gy: 1.6µm from CNRS LAAS,Toulouse, Franceup to 104 Gy: 0.25µm from REM, Oxford, UKup to 105 Gy: 0.13µm from REM, Oxord, UK


Online Measurement Techniques in Use Total Ionizing Dose

Response curves of RadFETs in use

1.6µm LAAS RadFET

0.01 Gy to 10 Gy

0.25µm REM RadFET

up to 104 Gy


Online Measurement Techniques in Use Non Ionizing Energy Loss

Non Ionising Energy Loss (NIEL) Measurement (1)

First Method: Bulk damage in forward biased p-i-n diode

NIEL causes bulk damage in silicon

⇒ reduced minority carrier lifetime in a p-i-n diode

⇒ increase of resistance

⇒ measure voltage change at given forward current

Φeq = k × (V − V0)

p-i-n diodes used in ATLAS:

1 108 to 1012 n/cm2: CMRP from University of Wollongong, Australia

2 1012 to 1015 n/cm2: OSRAM BPW34 Silicon PIN photodiode



Response curves of p-i-n diodes in use

CMRP p-i-n diode

108 to 1012 n/cm2

BPW34 p-i-n diode

up to 1015 n/cm2

The BPW34 diodes that are used in ATLAS were pre-irradiated with3× 1012 n/cm2



Non Ionising Energy Loss (NIEL) Measurement (2)

Second Method: Bulk damage in silicon→ Increase of leakage current (Ileakage) in reverse biased diode:

Φeq = Ileakage/(αV ) (V: sensitive (depleted) Volume)

1011 to 1015 n/cm2 higher fluences with higher voltagePad diode with guard ring structure on epitaxial silicon25µm thin → fully depleted at voltages < 30 V also after irradiation

Federico Ravotti


Online Measurement Techniques in Use Thermal Neutron Fluence

Thermal Neutron Fluence Measurements

DMILL transistors areused in readoutelectronics in parts of theInner Detector (SCT).

Base current at fixedcollector currentsensitive to fast andthermal neutrons:∆IbIc

= keqΦeq + kthΦth

keq and kth knownΦeq measured withdiodes→ determine Φth


Online Measurement Techniques in Use Thermal Neutron Fluence

Radiation Monitoring Sensor Boards

Inner Detector: 14 Modules that contain:

3 RadFETs for different dose ranges2 PIN diodes for low and high fluences1 Epitaxial (large fluence range)2 DMILL bipolar transistorsNTC temperature sensorresistive pad for heating on the back side

Outside the Inner Detector region: 48 modules

1 high sensitivity PIN diode (CMRP)1 RadFETNTC temperature sensor


Readout and Online Monitoring

Readout

Usage of standard ATLAS components for straight forwardintegration:

ELMB: 64 adc channels, CAN bus communicationELMB-DAC: current source, 16 channels

Sensors are only biased during readoutPVSS based detector control system (DCS)Integration in ATLAS DCS and data base archiving


Tests in Mixed Radiation Field

Tests in Mixed Radiation Environment at CERN PS

Mixed high energy particles in IRRAD6 environment at CERN PS.

Two modules (Inner Detector style) are irradiated since mid May.

Test of readout setup/procedure and calibration constants.

F. Ravotti, M. Glaser et. al



PIN diodes in Mixed Radiation Environment

Secondary Emission Counter (SEC) counts number of protons

conversion factors to TID and NIEL from previous measurementsnot useful for very small doses (unstable beam conditions)

CMRP PIN diode also sensitive to low fluences (109 1 MeV neq/cm2).

Good agreement between PIN diodes (20 % uncertainty).



RadFETs in Mixed Radiation Environment

High sensitivity RadFET

(LAAS 1.6µm)

Medium sensitivity RadFET

(REM 0.25µm)

LAAS RadFET sensitive already at doses 10−2 Gy

But: reduced response of LAAS in proton rich environment→ recalibration for this special case



DMILL Transistors in Mixed Radiation Environment

β = collector currentbase current thermal neutron fluence

Directly measure degradation of DMILL transistor performance.

Determine neutron fluence (using Φeq from PIN diode as input).

Improvement of the readout timing to avoid “noise”



ATLAS Baseline Measurements


Summary

Summary

Radiation monitoring important

to cross check simulationsdetermine the correlation between dose levels and luminositymonitor electronics performance changes - particularly in the innerdetector

The system in ATLAS allows online monitoring of radiation levels:

TID in SiO2 from cGy up to 100 kGyNIEL in Si from 108 neq/cm−2 up to 1015 neq/cm−2

thermal neutron fluence and degradation of DMILL bipolar transistors

Integration in ATLAS Detector Control System

Test and optimization in mixed radiation field at low dose rates


Summary

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Summary


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